Researchers Pin Slowdown in Tropical Pacific Flow on Climate Change

4 May 2006

The Walker Circulation spans almost half the circumference of the Earth. Click to enlarge. Source: Gabriel Vecchi, UCAR

The Walker circulation—a vast loop of winds that drives climate and ocean behavior across the tropical Pacific—has weakened by 3.5% since the mid-1800s, and may weaken another 10% by 2100, according to a study led by University Corporation for Atmospheric Research (UCAR) scientist Gabriel Vecchi.

Based on on their modeling, the researchers conclude that the slowdown is largely due to human-induced climate change. The findings are published in the 4 May issue of Nature.

The Walker circulation spans almost half the circumference of Earth and pushes the Pacific Ocean’s trade winds from east to west, generating massive rains near Indonesia, and nourishing marine life across the equatorial Pacific and off the South American coast. Changes in the circulation, which varies in tandem with El Niño and La Niña events, can have far–reaching effects.

Another view of the Walker Flow. Click to enlarge. Source: Gabriel Vecchi, UCAR

The Walker circulation takes the shape of a loop with rising air in the western tropical Pacific, sinking air in the eastern tropical Pacific, west-to-east winds a few miles high, and east-to-west trade winds at the surface. The trade winds also steer ocean currents. Any drop in winds produces an even larger reduction in wind-forced ocean flow—roughly twice as much in percentage terms for both the observed and projected changes, says Vecchi.

Several theoretical studies have shown that an increase in greenhouse gases should produce a weakening of the Walker circulation. As temperatures rise and more water evaporates from the ocean, water vapor in the lower atmosphere increases rapidly. But physical processes prevent precipitation from increasing as quickly as water vapor. Since the amount of water vapor brought to the upper atmosphere must remain in balance with precipitation, the rate at which moist air is brought from the lower to the upper atmosphere slows down to compensate. This leads to a slowing of the atmospheric circulation.

Based on observations since the mid-1800s, the paper reports a 3.5% slowdown in the Walker circulation, which corresponds closely to the number predicted by theory. To establish whether human-induced climate change is at work, Vecchi and colleagues analyzed 11 simulations using the latest version of the GFDL climate model spanning the period 1861 to 2000.

Some of the simulations included the observed increase in greenhouse gases; others included just the natural climate-altering factors of volcanic eruptions and solar variations. Only the simulations that included an increase in greenhouse gases showed the Walker circulation slowing, and they did so at a rate consistent with the observations.

Based on the theoretical considerations, and extrapolating from their 1861–2000 analysis as well as from other simulations for the 21st century, the authors conclude that by 2100 the Walker circulation could slow by an additional 10%. This means the steering of ocean flow by trade winds could decrease by close to 20%.

This could have important effects on ocean ecosystems. The ocean currents driven by the trade winds supply vital nutrients to the near-surface ocean ecosystems across the equatorial Pacific, which is a major fishing region.

—Gabriel Vecchi

Simulation results depend on the assumptions and conditions within different models. However, the agreement of theory, observations, and models for the past 150 years lends support to this outlook, say the authors.

The study sends mixed signals on the future of the El Niño–Southern Oscillation—the system of ocean-atmosphere linkages that produces the worldwide weather of El Niño and its counterpart, La Niña.

The circulation has been tending to a more El Niño-like state since the 1860s. However, the dynamics involved here are distinct from those of El Niño.

Comments

Do they have modeling that explains the mini ice age from the fourteenth to the nineteenth century.The world entered the cooling period within a decade and in the end transitioned to a warmer trend just as quickly.
Is it possible that after a five hundred year cool cycle we have entered a warm cycle of undetermined length?Recent measurements show that Mars is also warming.This would seem to point to a sun cycle.We are currently entering a very active sunspot activity period.
I believe that man has contributed to the cycle but asking questions of connventional wisdom should not reflexively be rejected as heresy.
In the seventies I read of several studies warning of the impending ice age.In the early twentieth century scientists consumed radioactive material believing it had a rejuvenating effect.
When they project out a hundred years do they assume fossil fuel use for a hundred years?Do they enter an x factor into some models that assume we make extensive technological achievments by 2050?
I have heard contradictory studies labeled as tainted because they got funding from energy companies.Are university studies tainted by the grant application process.Several peer reviewed studies{not climate} have been discovered to be fraudulent recently{cloning in S.Korea}.
Its curious to me thst weather forecasts three days out can be often wrong yet the temperature rise can be confidently predicted a hundred years out. I read anthropological and cosmological theories by a phd that is then immediately savaged by three other scientists who think he is all wet.It would seem that climate prognostication would have the same uncertain data inputs as these fields yet there is near unanimity.
While I ask these questions I seek energy efficiency in my day to day life.I have brought research from this site to my company and other local businesses.Cleaner air and secure energy supplies are appealing to me.
Signed:The happy heretic

When theory and practice agree there is a reasonable chance you are on to something. (If it quacks like a duck and walks like a duck!)

Regardless of whether solar flares or CO2 are causing the increase, the results will be equally profound. The real question is, what should our response be. Will we prepare or choose the status quo?

In 20 to 30 years when the oil is gone, will we have found a cleaner source of energy or will we tap into the mighty coal reserves of America and China in panic, desperation, and haste. Coal will likely be the saviour of industrialisation, but will we have done the research to use it cleanly? Thirty years ago during the first oil crisis would have been a good time to start preparing. Tempus fugit.

We know Mars is warming up, but we're unceartain if earth is? Interesting. :D When theory and practice agree I'd say there is a reasonable chance of a match.

Regardless of whether solar flares or CO2 are causing the increase, the results will be equally profound. The real question is, what should our response be. Will we prepare or choose the status quo?

In 20 to 30 years when the oil is gone, will we have found a cleaner source of energy or will we tap into the mighty coal reserves of America and China in panic, desperation, and haste. Coal will likely be the saviour of industrialisation, but will we have done the research to use it cleanly? Thirty years ago during the first oil crisis would have been a good time to start preparing. Tempus fugit.

First, counter to what the authors of this research (or any in the global warming community) claim, there is no real evidence that anthropogenic causes are the source for the observed warming. All there that there is are models which are consistent with the rising temperatures, but that is no proof. Note that even the sign of the anthropogenic effect is not known for certain! More about it at this discussion.

Second, a more specific point, the authors conclude it is a human induced effect because:

"Only the simulations that included an increase in greenhouse gases showed the Walker circulation slowing, and they did so at a rate consistent with the observations."

This is because the simulations which included solar forcing included the direct effect of a variable solar irradiance. This effect is small and of course cannot explain the global warming (and obviously, neither the change in the Walker circulation). The effect which they should have taken in their simulation, but didn't is the indirect solar activity → cosmic ray flux → climate link which can explain a significant part of the warming, and may (or may not) explain the change in the Walker circulation.